There are others too like "nanakyo" and hachikyo etc etc. They are not even named most of the times but only refered to as henka wasa of ikkyo or jonkyo etc.

Rokkyo however is probably a borderline general kihon. One can see both 2nd and 3rd doshu do this frequently against jo tsuki etc.

A try to explain it: Take a karate like tsuki at chest level delivered with ukes right hand (your left side). Sidestep slightly to your left and meet the punching arm from the side with your left hand( like you were about to do kote gaeshi but more to the outside of ukes hand instead of on top of it).

Extend the punch forward and down while you do a tenkan (again kind of like kote gaeshi).

At the same time let your right hand, palm up, grab the punching hand from below.

Now, when ukes hand has been guided down and to the side (thus taking ukes balance), let your left hand grip roll away from your belly and your right hand roll towards your belly, rotating ukes arm in the process, exposing his elbow toward you.

This is done at the same time as the tenkan is completed, your lower left arm making full contact with ukes arm, your hand now leading ukes hand up, but with the contact of your left arm with ukes arm keeping his elbow low, thus creating a lever to lower ukes whole body down.

The pressure should be to the side of ukes elbow i.e. 90 degrees angle away from straight in to the elbow oposite how the elbow bends.

The end position should be something like you lowering you hip to secure the downwards pressure on the side of ukes elbow. Ukes arm should be pointing almost verticaly up. You maintaining the grip with both hands at about chest height. Your left foot forward.

Hard to do, quite dangerous to train fast, but fun to train.

O, btw. I think I've heard this one being refered to as waki gatame too.